Space heaters catastrophically failing

[Electromatic]

I went to an industrial site the other day where they have recently had several large, 3Ø, 480V, 13.5kW, plug-in space heaters blow out an element. The same element failed in the different heaters. The service is a corner-grounded delta, and the grounded phase was one leg of the failed elements. The receptacle they plug this into is tapped directly off the main 400A breaker of a motor control center. There is a switch for the receptacle but no separate overcurrent protection. There is a small-ish AB SMC Flex drive fed off the MCC, and there's also a ≈1yr old larger Siemens Sinamics frequency converter fed off it.

The site is having the power company monitor their service for abnormalities. We couldn't find anything out of the ordinary. We are suggesting a separate OCPD for the heater receptacle. Does anyone have any thoughts about what might be causing this or what to look for? Could something be going awry with the grounded leg? Could the drive or newish frequency converter be dumping energy back through the MCC and into the heater receptacle? I didn't see any line reactors, resistors, etc.

Thanks in advance for any ideas.

 

[petersonra]

I went to an industrial site the other day where they have recently had several large, 3Ø, 480V, 13.5kW, plug-in space heaters blow out an element. The same element failed in the different heaters. The service is a corner-grounded delta, and the grounded phase was one leg of the failed elements. The receptacle they plug this into is tapped directly off the main 400A breaker of a motor control center. There is a switch for the receptacle but no separate overcurrent protection. There is a small-ish AB SMC Flex drive fed off the MCC, and there's also a ≈1yr old larger Siemens Sinamics frequency converter fed off it.

The site is having the power company monitor their service for abnormalities. We couldn't find anything out of the ordinary. We are suggesting a separate OCPD for the heater receptacle. Does anyone have any thoughts about what might be causing this or what to look for? Could something be going awry with the grounded leg? Could the drive or newish frequency converter be dumping energy back through the MCC and into the heater receptacle? I didn't see any line reactors, resistors, etc.

Thanks in advance for any ideas.

Each heater should probably have a 20 Amp OCPD.

I doubt that the drives are the source of the problem. Sounds like they had an excursion above 480 V to ground that caused the heater element to overload.

 

[ptonsparky]

Sounds like they wanted heat at all costs. They got it.
No SC or OC protection...sheesh.

 

[synchro]

Do the heating elements have a metal sheath around them like Calrods do? If so, then depending on the construction of the heater, a corner grounded delta might put more voltage stress on the magnesium oxide insulation inside of the element than a grounded wye configuration would.

I don't know if this is applicable in your case, but the following are from a manual on Fostoria portable infrared heaters:





Real marketing speak saying that "these precision characteristics can change with time."